1. Field of the Invention
This invention relates to techniques for fabricating integrated circuit devices, and more particularly to techniques for using highspeed electron beams to fabricate image pattern areas requiring large amounts of digital data at a high rate of speed. Specifically, the invention relates to a data interface buffer system capable of delivering pattern data to an electron beam tool from either a pattern data storage library via a channel or a computer processing unit with minimal processor intervention.
2. The Prior Art
The use of electron beam exposure has been recognized as an important tool for integrated circuit fabrication techniques for many years. Early attempts to use electron beam exposure techniques typically required the dedication of an entire computer system in order to provide both data and control information to an electron beam column. See for example, the article by S. Miyauchi et al, "IC Pattern Exposure by Scanning Electron Beam Apparatus", Solid State Technology, July 1969, pp. 43-48, in which an electronic computer provides the interface between an electron beam column and its data input. The data transfer between the column and the computer being such as to require a high degree of interactive control. Several additional similar computer controlled systems are represented by U.S. Pat. No. 3,820,894 to Hyatt; the article, "An Electron Beam Maskmaker", by J. P. Beasley et al, IEEE Tr. Electron Devices, Vol. ED-22, No. 7, July 1975, pp. 376-84; and the article "A New EB Microfabrication System", Solid State Technology, August 1975, pp. 63-4. It was not unusual for such systems to require from one to several hours to expose the integrated circuit pattern of a single semiconductor wafer. All of these early systems relied on a raster scan technology in which data was required to be serially supplied continuously to the electron beam column as the beam traced patterns within a small field. In order to expose larger areas it was necessary to use a step-and-repeat technique in which the wafer or workpiece was mechanically incremented to a field adjacent to that previously exposed.
An improvement to the raster scan step-and-repeat exposure technique is described in U.S. Pat. No. 3,900,737 to Collier. In this technique a continuous stream of data is serially applied to the electron beam column while the workpiece is continuously moved under the beam, thus providing a long strip of exposed area. A typical pattern data input control for such systems includes a pair of shift registers, each having a data bit capacity equal to that of a single raster scan, which are alternately used to accept data from the computer supplying data and to transfer data to the electron beam column. Such systems are throughput-limited by the data transfer rate between the magnetic tape containing the predetermined pattern data and the computer and by the data transfer rate between the computer and the shift registers, both of which are directly under control of the computer.
A further improvement in data transfer rate is taught in U.S. Pat. No. 4,063,103 to Sumi. Here the main core memory of the computer is divided into two independent sections so that pattern data can be read from one section to the shift registers while additional pattern data is being read from a magnetic disk to the computer memory. A further improved raster scan electron beam exposure system was described by Sumi et al at the 10th Conference on Solid State Devices, Tokyo, 1978. This paper, Japanese J. Appl. Phys., Vol. 18 (1979) Supplement 18-1, pp. 303-309, describes a pattern data transfer system, similar to that in U.S. Pat. No. 4,063,103, which includes a separate memory under control of a dedicated mini-computer system. Data is supplied to the memory from a magnetic disk on a demand basis and from the memory to a Direct Memory Access unit which includes the shift registers for controlling the electron beam column. The data transfer rate between the magnetic disk is less than the time required by the electron beam column to use the same amount of data, thus ensuring a continuous flow of pattern data to the electron beam column. Although disk-stored pattern data is subject to a data compaction scheme to reduce the amount of data to be stored and thus reducing the effective disk to storage data transfer time, the maximum data transfer for this system is 20,000,000 bits (20 M bits) per second. This data rate requires about one hour to expose an integrated circuit pattern of 2 micron features on a 100.times.100 mm.sup.2 substrate.
A second electron beam exposure technique is referred to as a vector scan technique, the general characteristics of which are described in the paper presented by T. H. P. Chang at the 8th Conference (1976 International) on Solid State Devices, Tokyo, 1976, see Japanese J. of Appl. Phys., Vol. 16 (1977) Supplement 16-1, pp. 9-16. In the vector scan technique small geometrical shapes, usually rectangles, are traced by the electron beam after being first positioned to a reference point on the surface to be exposed. The vector scan technique enables the quantity of pattern data to be reduced because only the location, shape and size of the rectangle are required for each subunit of the pattern. A step-and repeat technique is used to move the workpiece from one area to another, each of which may contain a number of rectangles. The article "Automation of Vector Scan I Electron Beam Lithographic System", A. D. Wilson et al, Proceedings of the Symposium on Electron and Ion Beam Science and Technology, Seventh International Conference, Washington 1974, pp. 361- 376, describes the system features of a particular version of a vector scan system. Here an IBM 1130 Processor having a 32K word core memory is used as a temporary data storage buffer for pattern data initially stored on a magnetic disk. Data from the magnetic disk is transfered to one of an A or B section of core storage over a high speed Storage Access Channel (SAC) which operates under control of the 1130 on a cycle steal basis. That is the 1130 is interrupted during any SAC transfers. Data from the core memory is transferred to a cache memory having a capacity of 256 words at a transfer rate equal to that of the cycle time of the core memory, 2.5 microseconds per word. After initial loading of 256 words into the cache memory, further data transfers are not made until the cache word count is reduced by the pattern generator to 190 words at which time the cache is refilled. Data can be fetched by the pattern generator from the cache in about 0.4 microseconds per word. For the 16 bit word used, the maximum data transfer rate is about 40 M bits per second, or about twice that of the above second described system of Sumi.
The article, "Vector Scan I, An Automated Electron Beam System for High Resolution Lithography", T. H. P. Chang et al presented at the same conference as that of Wilson, above, at pp. 392-410, is of interest as it discusses various operating parameters and limitations of electron beam exposure systems. Two items in particular are pertinent. One is that electron beam exposure systems are data throughput-limited and the other is that the beam hardware is capable of operating at much higher data rates than that of the above systems. In particular, beam stepping rates of at least 30 MHz and up to several thousand MHz can be achieved.